Sorry if this a duplicate,. I tried the search feature and found a few entries on the topic of sizing this, but none with any specific examples.
I am a new customer trying to deploy VBO for the first time. I have also review documentation, but it seems to gloss over this topic.

I have also review VBO deployment guide, which is for 2.0 which may be outdated since the relase of 3.0.

Using our mailbox size of 215GB and daily change rat of .1 GB and 90 days of retention, I come to a requirement of 19TB.
Does this sound right to anyone else? that seems quite high!

First, the sizing recommendations provided in the v2 deployment guide are still relevant for v3. The updated version should be published in a couple of weeks, but for estimating Exchange Online backup storage the current guide is OK.

Next, in the VBO PowerShell module, there's a cmdlet which allows calculating the initial storage requirements - and it's also available in the Community Edition, which you can install and use for free. For example, to estimate the first full backup size of all mailboxes with Snapshot-based retention, you could run the following: